The technology for production of circuit boards for interconnecting electronic components has advanced considerably in recent times. The advance in integrated circuit technology has brought about ever increasing densification of electronics which in turn has brought about an ever increasing demand for denser and more reliable circuit boards for interconnecting the components.
According to one highly successful technique, dense circuit boards are created by scribing or writing wires on the surface of the board using very fine insulated copper wire. The wires are deposited according to a computer generated program. The wire pattern is thereafter encapsulated and the ends of the wires are connected to terminal pads on the board surface. The technology is generally described in U.S. Pat. Nos. 3,674,602 and 3,674,914. One of the significant advantages of the wired circuit boards over conventional printed circuit technology is that the insulated wires can cross one another and therefore very dense connection boards can be made in a single layer thereby eliminating the need for interlayer connections.
In the past, the connections to terminal pads in the wired circuit boards has usually been accomplished by plating. After the wire pattern is deposited and encapsulated, holes are drilled through the board at appropriate locations and then plated. The hole plating is done in a manner that not only plates the hole and forms the terminal pad, but also so that the end of the insulated wire exposed by the drilled hole is electrically connected to the pad.
Soldering techniques have, of course, long been used to connect wires to terminals. However, such soldering techniques have generally not been regarded as useful in high speed automatic production of circuit boards because of the difficulty in keeping the solder joint localized, because of the need to avoid heat damage to the plastic board substrates, and because of the danger of solder entering the holes associated with the terminal pads.
There have been prior attempts at solving these problems, such as:
(a) configuring the solder pads to provide a narrow heat transfer restriction, between the solder area and the plated hole (Stranco U.S. Pat. No. 3,673,681);
(b) providing an electrically conductive, heat resistive nickel layer under the solder regions (Stranco U.S. Pat. No. 3,673,681);
(c) cooling the soldering area with the flow of air or inert gas (Stranco U.S. Pat. No. 3,673,681, Larsen U.S. Pat. Nos. 3,650,450 and 3,812,581);
(d) prestripping the segment of wire to be soldered so that the solder joint need not be subjected to high temperatures required for insulation stripping (Nicholas U.S. Pat. No. 4,031,612);
(e) using parallel gap soldering where the heat is generated at the solder surface by using the solder pad to complete an electric heat generating circuit (Mulchay U.S. Pat. No. 3,444,347); and
(f) controlling the heat generated by using temperature measurements to control the electric current generating the heat (Denney U.S. Pat. No. 3,778,581).
Although by using a combination of the above teachings it is possible to produce satisfactory solder joints, these techniques do not provide a system capable of tolerating the range of variations and conditions encountered in commercial production. Also, some of the methods according to the prior art techniques enumerated above add considerable cost and complexity to the operation or require special board configurations which reduce the board's surface area available for routing wires.